The invention relates to a location system capable of deriving calibration information from location signals received from a device to be located by the system. In particular, the calibration information defines a time offset in the clock of each receiver of the system.
Infrastructure-based location systems use items of equipment placed at known points in the environment to determine the positions of other objects in that environment. Uses of such systems include the tracking of pallets within warehouses, finding colleagues in a workplace environment, and monitoring the positions of soldiers during a training exercise.
Typically, these location systems operate by placing a tag transmitter device on the object(s) to be tracked. These tags transmit signals which allow the tag location, and hence the object location, to be determined. Typically, the signals are coded in some way to allow the positions of different tags in the same space to be determined. The signals may be radio waves, light waves, ultrasonic waves, etc. The infrastructure consists of a set of base units, each having a receiver for determining particular qualities of the incoming signals from tags that allow the tag location to be determined.
In many location systems, the receivers detect the time-of-arrival (ta) of the incoming signal. If the receiver knows the precise time-of-transmission of the signal (tt), and the speed of the signal through the environment (v), it is possible to determine the distance (d) between the receiver and the tag, using the equationd=v(ta−tt)
As mentioned above, the positions of the receivers (i.e. the base units) are known, and if four or more distances from different non-coplanar receivers to the tag can be found in this way, then a unique 3D position for the tag can be found using a process known as trilateration (closely allied to the better-known process of triangulation). Systems that work in this way are known as time-of-arrival (TOA) systems, and a TOA system based on ultrasonic techniques is described in the paper “A New Location Technique for the Active Office”, A. Ward, A. Jones, and A. Hopper. IEEE Personal Communications, 4(5):42-47, October 1997.
In fact, typically the receivers in the location system do not know the precise time-of-transmission of the signal from the tag. Achieving synchronisation of the tag with the infrastructure requires expensive and power-hungry circuitry at the tag and this is normally in conflict with the requirement that the tag should be unobtrusive, inexpensive and battery-powered (with a very long battery life). Therefore, a different approach called time-difference-of-arrival (TDOA) location may be used.
In a TDOA location system, there is no need for precise synchronisation of the tags and the receivers. Instead, all elements of the receiver infrastructure are precisely synchronized, using stable clocks at each receiver that are periodically corrected via some wired or wireless reference timing signal that is distributed to the receivers. If the receivers are synchronized (so that an event determined by one receiver as occurring at time t would also be determined by all other receivers as occurring at time t) then it is possible to find information about the tag's position by comparing the differences of the signal's time-of-arrival at multiple receivers. For example, suppose the signal was transmitted at an unknown time tt and received at times ta1 and ta2 at receivers 1 and 2 respectively. Then, we can find the difference, dΔ12, between the distances from receiver 1 to the tag and receiver 2 to the tag, as follows:dΔ12=v(ta−tt)−vx(ta2−tt)=v(ta1−ta2)
As can be seen, the (unknown) time of transmission is not required to determine the distance difference dΔ12.
Since the positions of the receivers are known, this equation describes a hyperboloid of revolution, on which (at some point) the tag may be found. By repeating this process with other receiver pairs and determining the locus of points where the derived hyperboloids of revolution intersect, the possible 3D position of the tag can be narrowed down. If four or more non-coplanar receivers detect the signal from the tag, then the intersection of the derived hyperboloids of revolution will represent a unique solution for the tag's 3D position. One implementation of a TDOA location system is described in the paper “Commercialization of an Ultra Wideband Precision Asset Location System”, R. J. Fontana, E. Richley, J. Barney, Proceedings of the 2003 IEEE Conference on Ultra Wideband Systems and Technologies, November 2003, Reston, Va.
In some location systems, the receivers may detect the angle-of-arrival (AOA) of the incoming signal from the tag. One way (but not the only way) for the receiver to accomplish this is to look for phase differences of the incoming signal at multiple antennas in the receiver unit. For example, a single receiver unit with four coplanar (but non-linear) antennas could get a 2D bearing for the tag in azimuth and elevation. By combining the 2D bearing information from two or more receiver units, a 3D position for the tag can be computed. Note that no synchronisation between receiver units is required to use AOA location techniques, although both the position and orientation of the fixed receiver units must be determined when the infrastructure is surveyed.
The TDOA and AOA techniques may be combined, for greater system robustness. When both techniques are used, a computer system gathers all the available TDOA and AOA data from the receivers that detected a tag's signal and uses them to compute a solution for the tag position best matching the input data. The system may attribute different weights to each item of data used in the position calculation.
Some infrastructure-based location systems use both TDOA and AOA techniques. Clocks at different receivers in such a hybrid location system also need to be synchronized because, as noted above, if TDOA location techniques are to be used the receivers in the location system infrastructure must be synchronized.
One method of achieving synchronisation between the receivers is to equip each receiver with a perfect clock. If these perfect clocks were gathered in one place, adjusted so that all were exactly synchronized and taken to the receivers, then each receiver could determine the time of arrival of signals in the common receiver time frame using its (now local) clock.
In practice, this approach would have a number of problems. First, perfect clocks do not exist. Real clocks drift relative to each other and therefore even if the clocks were adjusted to be exactly synchronous at one point in time, they would start to drift out of synchronisation. Second, it might well be difficult to move the clocks to one point in space so they could be synchronised locally.
Both problems can be solved by using a clock at each receiver which is periodically corrected by an external timing reference signal. If the clock at the receiver is stable enough and/or the corrections via the external reference signal are frequent enough, then the drift in the receiver's clock relative to those of the other receivers can be kept sufficiently low that the impact of any such drift on the location performance of the system is negligible.
Typically, the external timing reference is generated using a single clock located at one point and the signal from that clock is distributed, via wires or wirelessly to each of the receivers. In some cases, the external timing reference signal arriving at a receiver may be used to make an adjustment to a local clock on the receiver, or it may be used directly as the local clock.
Transmission of the external timing reference signal to the receiver is not instantaneous and this propagation time may well need to be taken into account, particularly with systems where the measurement signal transmitted by the tag is electromagnetic. In these cases, a very small clock offset at one of the receivers can translate into a large positioning error. This propagation time, represents an extra unknown (one for each receiver) in the system ├ it may be accounted for directly (if known) by suitably adjusting the local clock of the relevant receiver (if the local clock mechanism accommodates a mechanism for making such a change), or more commonly it may be accounted for during the position calculation process where propagation times represent further unknowns (one for each receiver) in the system of equations relating measurements to the position of the tag.
In situations where the speed of propagation of the external timing reference signal vp through its distribution medium is known, one way of determining the propagation delay tp from a fixed external timing reference signal to each receiver is to determine the physical distance dp from the reference source to each receiver and to calculate the propagation delay as follows:tp=dp/vp 
U.S. Pat. No. 6,054,950 describes a TDOA location system in which an ultra-wideband radio synchronisation pulse is transmitted from a reference station placed at a known point to a set of receivers also placed at known points in the environment, allowing the local clocks at each receiver to be globally synchronised with other receivers via the above mechanism. However, this approach has the disadvantage that not only must the positions of the receiver units be known in advance of system operation, so must the position of the external timing reference signal generator.
An alternative synchronisation scheme involves distribution of the external timing reference signal over a wired network. Here, the propagation delay depends on the length of the wires distributing the signal to each receiver (which may not be known because it may be desirable to route the signal over an existing network within a building and it may not be possible to directly measure the cable lengths) and the type of cable. However, even in situations where it is possible to use some external mechanism to determine the propagation delay along the cable at one point in time, temperature changes in the building can cause the properties of the cable (such as its length) to vary significantly, altering the propagation delay.
There is therefore a need for an improved scheme for calibrating a location system.